The presence in synaptic membranes of high affinity recognition sites for benzodiazepines capable of eliciting biochemical, physiological or behavioral responses when acted upon by specific ligands, prompted the search for an endogenous ligand that operates in physiological conditions. Recently, we developed a relatively simple procedure to isolate and purify this peptide to homogeneity. The peptide was termed DBI (diazepam binding inhibitor) and its extraction was routinely carried out using as a starting material rat a human brain homogenized in hot (80 degrees) I N acetic acid. Studies with the antibody reveal that DBI is unevenly distributed in brain and behavioral studies indicate that DBI acts like betacarboline in facilitating punished behavior. Trypsin digestion of DBI within the DBI molecule an octadecaneuropeptide, ODN (1-18), which has biological activity stronger than that of DBI itself. These data suggest that DBI may function as a precursor od a putative endacoid of benzodiazepine recognition site.